Conventional refrigeration systems reduce the temperature of commercial and residential spaces, such as homes, offices, commercial freezers, and refrigerated delivery trucks. Such systems typically operate on the vapor-compression cycle and include four major components: a compressor, a condenser, an evaporator, and an expansion valve.
In conventional operation, refrigerant is compressed by the compressor and exits the compressor as a vapor at a temperature higher than the inlet temperature. The vapor is then condensed by the condenser, turning the vapor into a liquid. The expansion valve rapidly decreases the pressure of the liquid refrigerant, resulting in a mixture of liquid and vapor at a lower temperature and pressure. Next, the refrigerant passes through the evaporator. A fan typically blows relatively warm air, from the space being cooled or refrigerated, across the evaporator. As the warm air passes the evaporator, and more particularly, the fins or coils of the evaporator, the warm air vaporizes the refrigerant in the evaporator since heat from the air is transferred to the refrigerant in the evaporator. And the refrigeration cycle repeats. In this way, the temperature within the space to be cooled is reduced.
One drawback of such conventional systems is that frost tends to build up on the evaporator when moisture condenses out of the relatively warm air and freezes on the outside of the relatively cold evaporator. This happens mostly on the fins or coils of the evaporator. To reduce or eliminate such frost, conventional systems typically include a defrost operation mode, where the evaporator is heated so that its surface temperature is above the freezing point of water. In that way, frost on the evaporator is melted and the resulting water is either blown off by a fan or drips off of the evaporator, thus eliminating the frost and the condensed moisture.
Typically, conventional defrost modes are periodically initiated by a timer. In such systems, the defrost mode generally ends when the temperature of the evaporator increases to a certain level, such as a few degrees above the freezing point of water. The temperature of the evaporator may be read, for example, by a thermostat. Other conventional defrost systems use an infrared source to direct infrared radiation through the region where frost would accumulate on the evaporator. The radiation is received by an infrared detector on the opposite side of that region. In such systems, the infrared detector can determine if frost is present by detecting the presence or absence of infrared energy.
But there are shortcomings with the conventional systems. For example, since such systems initiate the defrost mode based on a timer, the systems might engage the defrost cycle even if no frost is present on the evaporator. This can lead to an unnecessary and inefficient use of electrical power. Also, infrared systems are typically expensive and require a large amount of labor to install and maintain because of their complexity.
Embodiments of the invention address these and other issues in the prior art.